How to Calculate Your FIRE Number

The FIRE (Financial Independence, Retire Early) movement has one central goal: accumulate enough assets that your investment returns cover your living expenses indefinitely. The number that represents this threshold is what we call your FIRE number.

What Exactly Is Your FIRE Number?

Your FIRE number represents the total portfolio value at the point when you can safely withdraw funds each year without depleting your principal over your expected lifetime. It’s not about being rich—it’s about having enough.

The classic formula is deceptively simple:

FIRE Number = Annual Expenses ÷ Withdrawal Rate

But let’s unpack why this matters and how to think about each component.

The Components Explained

Annual Expenses

This isn’t your current spending—it’s your retirement spending. Key considerations:

• Current expenses are a starting point, but your spending will likely change
• Healthcare costs often increase (though Medicare helps at 65)
• Travel and hobbies might increase in early retirement
• Some expenses (commuting, work clothes) may decrease
• Consider potential family changes (kids, empty nest)

Rule of thumb: Use a conservative estimate. Most people underestimate by 15-25%.

The Withdrawal Rate

The withdrawal rate is the percentage of your portfolio you plan to withdraw annually. This is where FIRE gets interesting.

The 4% Rule

William Bengen’s 1994 research (later confirmed by the Trinity Study) suggested that a 4% withdrawal rate survived 30-year retirement periods ~95% of the time with a 50/50 stock/bond portfolio.

FIRE Number = $50,000 ÷ 0.04 = $1,250,000

Why 4% Works (Usually)

Historical US market returns suggest:

• ~7% average annual return before inflation
• ~3% average inflation
• ~4% “real” withdrawal rate

The Problem with 4%

The 4% rule assumes:

• 30-year retirement (you’re FIRE if you’re 35? That’s potentially 50+ years)
• Historical US market returns (future may be different)
• No significant portfolio fees
• Disciplined rebalancing

For longer retirements, many advisors suggest 3.3% or even 3%:

Withdrawal Rate	Safe for 40+ Years	$50k/year Requires
6%	~20 years	$833k
4%	~30 years	$1.25M
3.3%	~40 years	$1.5M
3%	~50+ years	$1.67M

Real-World Variables

Your actual FIRE number should account for:

1. Social Security/Pension: If you expect $20k/year in SS, you only need $30k from portfolio
2. Healthcare: Often the largest expense pre-Medicare
3. Geographic flexibility: Some locations allow much lower expenses
4. Part-time income: Even $15k/year reduces your needed portfolio by $375k at 4%

The Math: Compound Growth

Understanding compound growth helps set realistic expectations. Here’s how long it takes to reach your FIRE number:

Starting	Monthly	Return	FIRE Number	Years to FIRE
$50k	$2,000	7%	$1.25M	~14.5
$100k	$3,000	7%	$1.25M	~10.5
$0	$2,500	7%	$1M	~17

Try It Yourself

Use this calculator to determine your personal FIRE number:

Adjust the inputs to match your situation. Notice how small changes compound:

• +$100/month in savings = ~1 year less to FIRE
• -1% return = ~2-3 years longer to FIRE
• +$10k/year in desired income = +$250k FIRE number at 4%

Beyond the Number

Your FIRE number is a target, not a mandate. Consider:

• Coast FIRE: Save aggressively early, then “coast” with lower savings
• Barista FIRE: Part-time work to reduce portfolio needs
• Lean FIRE: Extreme frugality, lower number
• Fat FIRE: Traditional high savings, generous spending

Common Mistakes

1. Using current income instead of expenses: Your FIRE number is based on spending, not earning.

2. Ignoring taxes: Traditional 401k withdrawals are taxed. Roth is tax-free. Factor this in.

3. Not accounting for healthcare: One of the biggest variable expenses, especially before 65.

4. Being too optimistic on returns: 7% is the historical average—expect 5-6% real returns for planning.

The Takeaway

Your FIRE number is personal. The $1.25M figure you often see is based on $50k/year spending at 4%—that’s a comfortable middle-class lifestyle in most of the US, but not universally applicable.

Start with your actual expected expenses, apply a conservative withdrawal rate, and build from there. The number might be smaller than you feared, or larger than you hoped—but at least it’ll be yours.

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